The thing is, WB barely does anything with Bugs besides the yearly DVD release and the DC comics, while Disney puts Mickey all over the place. You can see Mickey as the shape of the logo, DVD releases, comics, preschool shows, and tons among tons of merchandise.
A lot of people (me included) prefer Bugs over Mickey when it comes to their cartoons. But when it comes to actual recognition from the average everyman, I think the answer to who the biggest icon is is pretty obvious.